Efficiency of SiC JFET-Based Inverters
Hui Zhang, Leon M. Tolbert
Min Kao Department of Electrical Engineering and Computer Science
The University of Tennessee
Knoxville, TN 37996-2100
Abstract—The state-of-the-art SiC JFETs are characterized.
Three-phase full-bridge inverter power loss models based on
experimental data are established and used to estimate inverter
efficiency. The impact of load power, temperature, and switching
frequency on inverter efficiency is analyzed and demonstrated.
The efficiency of the SiC JFET inverters based on present device
quality is above 98% with full load current, and more efficient
than most conventional Si inverters, especially at high
temperature and high frequency.
Index Terms—Silicon Carbide, JFET, inverter, efficiency,
power loss
I.
INTRODUCTION
Silicon carbide (SiC) power devices are expected to
substitute for Si counterparts in high-power, high-temperature,
and high-frequency applications due to their superior intrinsic
properties in these operating areas.
used as the anti-paralleled diodes for the SiC JFETs. Then, the
efficiencies of the two inverters are estimated based on the
device test results at different combinations of power,
switching frequency, and device junction temperature. The
impact of each factor on the efficiency of the SiC inverters is
analyzed and demonstrated.
II.
DEVICE CHARACTERIZATION
The two SiC JFETs listed in Table I are tested in this work.
The test method and test results for static and switching states
are presented respectively in the following paragraphs. The
diode is not tested because this work mainly focuses on
controlled switching devices. The datasheet values of the diode
were used in the simulations.
A. Static Tests
Curve tracer Tektronix 371B was used to obtain the
The first commercially available SiC power devices were
SiC Schottky diodes. They were introduced by Infineon in
2001 [1], and now are available from several manufacturers,
including Infineon, Cree, IXYS, Microsemi, and
STMicroelectronics, etc.
SiC JFET Tra nsfer characterstics
at diffferent tempera tures
SiC JFET forwa rd characteristics
at differe nt temperatures
12
5
4.5
25C
4
10
3.5
Ids [A]
Ids [A]
8
3
175C
2.5
25C - 175C
6
2
4
1.5
1
2
0.5
0
0
0
0.5
1
1.5
-20
2
-15
-10
-5
0
Vgs [V]
Vds [V]
(a) forward characteristics
(b) transfer characteristics
Fig. 1. Static characteristics of the SiC JFET (SiCED).
SiC JFET forward characteristics
at different temperatures
SiC JFET Transfer characterstics
at diffferent temperatures
5
8
4.5
7
25C
4
6
3.5
5
Ids [A]
3
Ids [A]
SiC controllable switching devices are available as
engineering samples such as JFETs from SiCED and
SemiSouth, MOSFETs from Cree, and BJTs from Cree and
TranSiC. These manufacturers will make these devices
commercially available soon, and SiC JFETs are likely the first.
To make good use of these devices and substitute for present Si
systems properly, it is necessary to learn the impact of power,
frequency, and temperature on a SiC-based power converter
system. In this work, the state-of-the-art SiC JFETs from
SiCED (normally on) and SemiSouth (normally off) are tested
for both static and dynamic characteristics. The losses during
conducting state and switching state are calculated, and loss
models based on curve fitting of the experimental results are
developed for the purpose of inverter simulations. Ratings
information for the tested devices is shown in Table I. The
devices are used as switching components to form two
inverters with the same ratings. More specifically, two SiC
JFETs are paralleled to achieve a higher current rating in the
inverter, and the SiC diodes (C2D101200 [2]) from Cree are
175C
2.5
2
25C - 175C
4
3
1.5
2
1
1
0.5
TABLE I. SIC SWITCHING COMPONENTS
0
0
0
Number
Type
Rating
Manufacturer
1
Normally-on SiC JFET
1200V/5A
SiCED
2
Normally-off SiC JFET
1200V/5A
SemiSouth
978-1-4244-2800-7/09/$25.00 ©2009 IEEE
0.5
1
1.5
Vds [V]
(a) forward characteristics
2
-15
-10
-5
0
5
Vgs [V]
(b) transfer characteristics
Fig. 2. Static characteristics of the SiC JFET (SemiSouth).
2056
ICIEA 2009
SiC JFET switching loss
SiC JFET switching loss
160
160
140
120
1A
100
2A
80
3A
60
4A
40
Energy Loss [1e-6 J]
140
Energy loss [1e-6 J]
forward and transfer characteristics. The SiC JFETs are put into
an environmental chamber to raise the ambient temperature up
to 175 °C in 25 °C increments. The characteristics of the
SiCED and SemiSouth devices are shown in Figs. 1 and 2,
respectively. As shown as Fig. 1 and Fig. 2, the on-state
resistance of the SiC JFETs increases with temperature from 25
°C to 175 °C, and transfer characteristics of the both devices
are nearly constant at this temperature range.
120
100
80
25
60
75
40
5A
20
125
20
175
0
0
50
100
150
0
200
0
dc
Thermal box
2
Temperature [C]
L
4
6
Current [A]
Fig. 6. Switching loss of the SiC JFET at different conditions (SiCED).
+
-
Fig. 3. Switching test circuit.
B. Switching Tests
The test circuit shown in Fig. 3 is used for switching tests.
It has a pure inductive load, and the switch is controlled by a
double-pulse signal. The current in the switch is controlled by
tuning the duty ratio of the first pulse. Commercial gate driver
IC HCNW3120 is used to drive these devices. Different power
supply voltages are given to meet the different gate signal
needs of the devices. The gate signal waveforms when using a
gate resistance of 10 are shown for SiCED and SemiSouth in
Figs. 4 (a) and Fig. 5 (a), respectively. In addition, the devices
are put into the chamber to raise the junction temperature up to
300
300
250
250
200
1A
2A
150
3A
4A
100
5A
50
Energy Loss [1e-6 J]
IF
SiC JFET switching loss
SiC JFET Switching loss
VF
Energy loss [1e-6 J]
DUT
200
150
25
100
75
125
50
175
0
0
0
50
100
150
200
0
2
Temperature [C]
4
6
Current [A]
Fig. 7. Switching loss of the SiC JFET at different conditions (SemiSouth).
175 °C. The switching waveforms of the devices at the test
condition of 200 V, 5 A and 175 °C are also shown in Figs. 4(b)
and Fig. 5(b). Their switching energy losses at different
temperature and different current level are shown in Fig. 6
(SiCED) and Fig. 7 (SemiSouth). All the switching losses
increase with increasing current and are almost constant with
increasing temperature.
III.
INVERTER POWER LOSS CALCULATION
Inverter power loss is mainly composed of conduction
power loss and switching power loss. A widely used averaging
technique [3-6] is employed to study the inverter power loss.
The primary algorithm is presented in this section. For more
details, please see [7-8].
(a) gate signal waveforms
(b) switching waveforms @ 175 °C
A. Conduction power loss
For SiC JFETs and MOSFETs, the conduction loss is
mainly caused by on-state resistance, Ron. It is calculated by
Fig. 4. Test waveforms of the SiC JFET (SiCED).
Pcond = I rms 2 ⋅ Ron
,
(1)
where Irms is the effective current flowing in the device. For an
inverter under SPWM control, the effective current can be
expressed as [8]
I rms = I
(a) gate signal waveforms
(b) switching waveforms @ 175 °C
1 1
+
M cos φ
8 3π
,
(2)
where M is modulation index, I is the peak of phase current,
and φ is the current phase angle with respect to voltage (The
Fig. 5. Test waveforms of the SiC JFET (SemiSouth).
2057
TABLE II. RESISTANCE PARAMETERS OF THE SIC JFETS
TABLE III. SWITCHING LOSS PARAMETERS OF THE SIC JFETS
Device
a
b
c
Device
d
e
f
SiC JFET (SiCED)
0.0021
0.5981
141.57
SiC JFET (SiCED)
0.9932
-3.6578
22.755
SiC JFET (SemiSouth)
0.0054
0.3457
107.43
SiC JFET (SemiSouth)
1.14
-1.9954
30.211
phase current is i = I sin(θ - φ), and only fundamental is
considered.). Therefore, the conduction power loss is
1 1
!
Pcond = I 2 Ron " +
M cos φ #
$ 8 3π
%.
where d, e, f are coefficients which are obtained from the curve
fitting of the experimental data in Figs. 6 and 7 (see Table III).
Then, substitute (6) into (5) to obtain (7), an expression of the
effective switching power loss based on test data.
(3)
Ron = aT 2 + bT + c (mΩ) .
(4)
The on-state resistances are shown in Fig. 8 and curve fitting
determines the parameters in (4). The a, b, and c coefficients of
the two devices are summarized in Table II. In addition, Fig. 8
also indicates that the conduction loss of the SiC JFET from
On-state re sistance s of the SiC JFETs
(7)
Effective switching energy loss of the SiC JFETs
140
350
Resistance [miliohm]
!
I # ( !W ) ,
%
The switching energy losses for the devices computed by (7)
are shown in Fig. 9. As shown in Fig. 9, the SiC JFET from
SiCED is more efficient in terms of switching losses.
Energy loss [1e-6J]
As shown in Fig. 1 and Fig. 2, the values of Ron are functions of
device junction temperatures. By letting the on resistance be a
quadratic function of current, it can be represented by
2d 3 e 2 f
Psw = f sw ⋅ Esw = f sw ⋅ "
I + I +
4
π
$ 3π
120
SemiSouth
100
80
60
40
SiCED
20
0
0
300
4
6
8
10
12
Current [A]
SiCED
250
2
200
Fig. 9. Effective switching energy loss of the SiC JFETs.
SemiSouth
150
100
0
50
100
150
IV.
200
Temperature [C]
Fig. 8. On-state resistances of the SiC JFETs.
SemiSouth is lower at room temperature, while at the
temperatures above 150 °C, the SiC JFET from SiCED is better
in terms of conduction loss.
B.
Switching power loss
By [7], the effective switching energy loss of a SPWM
controlled inverter is expressed as
Esw =
1
2π
2π +φ
,
& E (θ ) dθ
(5)
φ
if phase current is i = I sin(θ - φ). Further, the switching energy
loss is a function of current at a certain temperature by the test
results (see Figs. 6 and 7), and does not change much for
different temperatures. So neglecting the influence of
temperature, it can be expressed as a polynomial function of
current as
E (i ) = di 3 + ei 2 + fi (!J) ,
(6)
2058
INVERTER POWER LOSS CALCULATIONS
Based on the discussions in the previous section, the
efficiency for a three phase full bridge inverter based on each
of the two SiC JFETs is determined for different operation
conditions using MATLAB Simulink. Assume DC bus voltage
is 480 V, output voltage fundamental frequency is 60 Hz, and
the load is an RL load (R=23.81 , L=0.0015 H), which makes
current in the devices reach 10 A at modulation index M=1.
A. At different load powers
Assume the junction temperature is 50 °C and switching
frequency is 10 kHz. The modulation index is varied to change
the load current from 0 to 10 A, and the power losses for
different load currents are calculated. The efficiencies of the
inverters based on the two different devices are shown in Fig.
10. The efficiency increases with load power and reaches a
maximum at M=1. The maximum efficiency of each inverter is
99.0% for the SiCED JFET inverter and 98.8% for the
SemiSouth JFET inverter. Generally, the efficiency of Si
inverters ranges from 93% to 97% at rated power. So the two
SiC JFET inverters are more efficient than most Si inverters. In
addition, the losses of the SiC JFETs dominate the total
inverter losses, and unlike the diodes, the switching losses are
larger than the conduction losses in the two SiC JFETs.
Efficiencies of the SiC JFET inverters
at different power levels
1.00
Efficiencies of the SiC JFET inverters
at different temperatures
0.995
SiCED
Efficiency
Efficieny
0.99
0.98
SemiSouth
0.97
SiCED
0.990
0.985
SemiSouth
0.980
0.96
0.975
0.95
0
0.5
1
1.5
2
2.5
3
3.5
25
4
75
125
175
Temperature [C]
Load power [kW]
Fig.10. Efficiencies of the SiC inverters at different load powers.
Fig.12. Efficiencies of the SiC inverters at different temperatures.
B. At different switching frequencies
Assume the junction temperature is 50 °C and the load
current is 10 A. Increase the switching frequency to 50 kHz and
compute the power losses. As expected, the efficiencies of the
inverters decrease with the switching frequency as shown in
Fig. 11. The efficiencies of the two SiC JFET inverters
decrease slightly with increased switching frequency, and keep
relatively high values even at 50 kHz (97.1% for SiCED
inverter and 95.6% for SemiSouth inverter). The switching
frequency of 50 kHz is not possible for most high power Si
power devices due to the extreme high power loss and
demanding cooling requirements.
inverter can be substituted by a naturally or forced convection
heatsink depending on power involved.
V.
CONCLUSIONS
The SiC JFET inverters have relatively high efficiency at
all powers, switching frequency, and temperature ranges.
Compared to Si inverters, the advantage is more obvious at
high frequency and high temperature. Motion control, solar
energy, wind generation, and vehicle systems might benefit
from these SiC inverters in the near future. Finally, the SiC
JFETs presented in this paper are not commercial products.
Thus, with more development of SiC semiconductor
technology, the efficiency of inverters based on future SiC
products will be further improved.
Efficiencies of the SiC JFET inverters
at different switching frequencies
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1.00
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SiCED
0.99
[2]
0.98
0.97
SemiSouth
[3]
0.96
0.95
0
10
20
30
40
50
Switching frequency [kHz]
[4]
Fig.11. Efficiencies of the SiC inverters at different switching frequencies.
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2059
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